1. Field of the Invention
This disclosure relates to a method of manufacturing an integrated circuit device, and more particularly, to a method of manufacturing an integrated circuit device that includes a recessed channel transistor.
2. Description of the Related Art
As the integration density of integrated circuit devices increases, new technical problems to be solved emerge. One of these problems is that the channel lengths of transistors are becoming shorter. In the case of a planarized transistor, a channel length of the transistor is reduced as the integration density increases often resulting in a short channel effect (SCE). The SCE causes a punch through problem between a source and a drain resulting in a malfunction of the integrated circuit device, thereby reducing reliability of the integrated circuit.
Several suggestions for solving the SCE problem have been presented such as using a SOI substrate instead of a bulk substrate, a method of modifying the transistor by using a Fin Field Effect Transistor (Fin FET), or manufacturing the transistor with a three dimensional shape having a recessed channel (hereinafter, a recessed channel transistor).
FIG. 1 is a plan diagram illustrating an active region pattern A/P for forming a conventional recessed channel transistor and a gate pattern G. FIGS. 2A, 2B, and 2C are cross-sectional diagrams illustrating the conventional recessed channel transistor taken along lines A-A′, B-B′, and C-C′ in FIG. 1, respectively.
Referring to FIGS. 2A through 2C, a silicon substrate 10 is divided into a trench isolation region 40a and an active region defined by the trench isolation region 40a. A gate trench 90 is formed in the active region. A recess gate 98 buried in the gate trench 90 and source and drain regions 50 formed on both side of the recess gate 98 constitute a recessed channel transistor. A channel of the recessed channel transistor is formed along the sides and bottom of the gate trench 90; in FIG. 2A, the channel is formed from left to right, and in FIG. 2C, the channel is formed from front to rear. Accordingly, since the channel length of the recessed channel transistor is longer than that of a planarized transistor, problems associated with the short channel length can be reduced or solved.
However, as shown in the dotted circles in FIG. 2C, in the conventional recessed channel transistor a portion of the silicon substrate 10 referred to as a silicon fence remains between a side wall of the trench isolation region 40a and a side wall of the gate trench 90.
The silicon fence occurs because a vertical profile of the trench isolation region 40a has a predetermined slope close to the edge of the active region pattern A/P. The active region pattern A/P, defined by the trench isolation region 40a, has a narrower upper width d1 than a lower width d2 due to limitations of the dry etching process. That is, when etching the silicon substrate to form a trench, the side walls of the gate trench 90 have a predetermined slope. When forming the gate trench 90, in spite of efforts to form vertical side walls by etching the silicon substrate using an anisotropic dry etching method, it is unavoidable that a portion of the silicon substrate, that is, the silicon fence remains, on the lower edge of the active region.
Because of the silicon fence, the channel length of the recessed channel transistor in the central region of the active region pattern (refer to FIG. 2A) and in the edge of the active region pattern (refer to FIG. 2B) are different. When there is an especially short channel region in the recessed channel transistor, the threshold voltage of the transistor is not only reduced but a malfunction of the integrated circuit device results from a sub-threshold leakage current through the region. Moreover, when the silicon fence exists in the source and drain region, a short circuit between the source region and the drain region can occur.